Designers of vehicles commonly mount antennas on an outer surface of a roof of the vehicle. The roof or another planar surface of the vehicle acts as a ground plane for the antenna. Typically, the antenna is located in close proximity to a vehicle window. The performance of the antenna is proportional to the size of the ground plane. Increasing the size of the ground plane improves a radiation pattern of the antenna.
The optimal position for the antenna is in the center of the roof. The antenna can use the entire roof surface as a ground plane. The current trend in vehicle design is to conceal the antenna from view. The center of the roof, however, is a highly visible location. For aesthetic reasons, the antenna is often mounted at or near the edge of a vehicle surface, which reduces the effective size and symmetry of the ground plane. The positioning of the antenna in this manner degrades the performance of the antenna.
Some antennas are mounted at the edge of the roof surface in close proximity to a window. This location may allow radiation to propagate into the passenger compartment. To reduce the radiation into the passenger compartment and improve the radiation pattern of the antenna, a wire grid is located on the window adjacent to the antenna. The wire grid reduces radiation into the passenger compartment, and offsets the performance degradation caused by the asymmetrical ground plane.